Jeffrey Wright's (The Hunger Games) character Valentin Narcisse makes his first appearance in the clip. Narcisse is billed as "a Doctor of Divinity, philanthropist, student of culture and the man who runs Harlem".
Watch the Boardwalk Empire 'welcome back tease':
Steve Buscemi returns to his central role as the corrupt Nucky Thompson.
Other new cast additions alongside Wright include Office Space actor Ron Livingston, who will play a love interest to Gillian Darmody (Gretchen Mol) and former Wire actor Domenick Lombardozzi, who is cast as Chicago mobster Ralph Capone.
Patricia Arquette has also joined the cast as "tough-as-nails Tampa speakeasy owner" Sally Wheel.
Hyping the show's fourth season, HBO programming president Michael Lombardo said recently: "Terry Winter, Martin Scorsese and the rest of their outstanding team continue to produce a stunning show that never fails to surprise and entertain."
Boardwalk Empire will return in the US on Sunday, September 8 at 9pm on HBO. It is expected to air in the UK shortly after on Sky Atlantic.